The present invention relates generally to laboratory instruments which concentrate a volatile organic compound (xe2x80x9cVOCxe2x80x9d) sample obtained from a specimen of air, liquid, or soil to prepare such VOC sample for delivery to an analytical instrument such as a gas chromatograph (xe2x80x9cGCxe2x80x9d). During subsequent analysis, one or more detectors in the GC detects individual analytes from the VOC sample as the analytes sequentially exit the GC column. Such laboratory instruments are generally referred to as sample concentrators. The invention also relates to cryofocusing traps, sometimes referred to as cryofocusers, which can be used together with or as part of a sample concentrator and normally installed at the injection port of the GC for additional concentration (xe2x80x9cfocusingxe2x80x9d) of the VOC sample. The invention further has application to laboratory instruments known as autosamplers, which can include a sample concentrator or work in cooperation with one, and which have the capability to sample multiple specimens in an automated sequence.
The invention will be described primarily in connection with an air sample concentrator, but it can also be used with a liquid sample concentrator or a soil sample concentrator. In a liquid sample concentrator (see, e.g., EPA Method 502.2, rev. 2.0 (1989), or EPA Method 524.2, rev. 3.0 (1989), both incorporated herein by reference), the VOC sample is extracted (xe2x80x9cpurgedxe2x80x9d) from a sample matrix by bubbling an inert purge gas through an aqueous specimen. During this xe2x80x9cpurgexe2x80x9d mode, the inert purge gas, on its way to a vent, passes through a sorbent trap where the VOC sample is retained. Carrier gas during this mode flows through a carrier gas line uncoupled to the sorbent trap. In a subsequent xe2x80x9cdesorbxe2x80x9d mode, the carrier gas line is coupled to the sorbent trap, now at an elevated temperature, and the VOCs are swept out of the sorbent trap to the GC. A soil sample concentrator has a purge and desorb mode like the liquid sample concentrator, except that during the purge mode the purge gas typically bubbles through a mixture of the soil specimen and a liquid such as water.
As used herein, the term xe2x80x9cconcentrator trapxe2x80x9d refers to a device having an xe2x80x9cadsorbing statexe2x80x9d where VOCs collect or accumulate in the device, and a xe2x80x9cdesorbing statexe2x80x9dwhere the collected VOCs are released from the device. A sorbent trap comprising a tube filled with one or more layers of sorbent materials is one type of concentrator trap. A glass bead trap comprising a tube packed with glass beads, and temperature controllable to suitably low temperatures, is another type of concentrator trap. xe2x80x9cCryofocuserxe2x80x9d as used herein refers to a temperature controlled flow path capable of concentrating a VOC sample delivered to it and releasing the concentrated sample for delivery to a GC or other suitable analytical instrument. Although cryogenic temperatures (less than ambient room temperature, and usually less than about xe2x88x92100xc2x0 C.) and cryogenic cooling fluids (e.g. liquid nitrogen) are ordinarily used with cryofocusers, they may not always be required in view of unique testing circumstances or future design improvements. Cryofocusers tend to have a smaller internal flow path volume than concentrator traps, with typical volumes for a cryofocuser being about 0.01 to 1 cubic centimeters (cc) and typical volumes for a concentrator trap being about 3 to 30 cc.
Ordinarily, where a cryofocuser is used with a sample concentrator, the cryofocuser is fluidly coupled in series with a carrier gas line, and with the concentrator trap during the desorb mode of the sample concentrator. Afterwards, with the concentrator trap fluidly decoupled from the carrier gas line, carrier gas carries the focused VOC sample from the cryofocuser, which has been rapidly heated, to the GC. At all relevant times, a substantially constant flow rate of carrier gas flows through the carrier gas line to the GC for proper operation of the GC column and detector(s). A drawback associated with such arrangement is that the optimum flow rate for the GC is not necessarily the optimum flow rate for desorbing the analytes from the concentrator trap. Even where a split injection arrangement is used at the injection port of the GC, causing only a portion of the carrier gas flowing through the carrier gas line to flow through the GC, a proportional amount of the VOC sample is discarded and not available for analysis.
Improvements to sample concentrator performance are desired to enable users such as pharmaceutical or environmental testing laboratories to extend the detection limits of current technology and to increase efficiency.
According to one preferred embodiment, a sample concentrator has a carrier gas line, a concentrator trap, a multiple port valve, and a bypass valve. The carrier gas line as a proximal end fluidly coupleable to a source of carrier gas, and a distal end fluidly coupleable to an analytical instrument. The multiple port valve couples to the carrier gas line and to the concentrator trap, and has at least one state fluidly decoupling the carrier gas line from the concentrator trap and at least another state fluidly coupling the carrier gas line to the concentrator trap. The bypass valve fluidly couples to the carrier gas line to permit fluid flow away from the carrier gas line as a function of the state of the bypass valve. A controller is also provided, which directs operation of the multiple port valve and the bypass valve.
According to another preferred embodiment, a system for concentrating a VOC sample for delivery to an analytical instrument includes a flow splitter, a cryofocuser, a bypass line, and a valve. The cryofocuser has an inlet to receive the VOC sample and an outlet fluidly coupleable to the analytical instrument through the flow splitter. The flow splitter also fluidly couples to the bypass line, and the valve regulates gas flow through the bypass line.
A preferred method is disclosed for concentrating a VOC sample before delivery to an analytical instrument. The method includes loading a concentrator trap with the VOC sample, and transferring such sample to a cryofocuser with carrier gas flowing at a first flow rate. The VOC sample is subsequently released from the cryofocuser with carrier gas flowing at a second flow rate. Preferably, the first flow rate is greater than the second flow rate. During the transferring and releasing steps, the flow rate of carrier gas through the analytical instrument preferably remains substantially constant at or below the lesser of the first and second flow rate.
Another preferred method for focusing a VOC sample is disclosed, the method including providing a carrier gas flow through a cryofocuser, carrying the VOC sample to the cryofocuser at a first carrier gas flow rate, and releasing the VOC sample from the cryofocuser for delivery to an analytical instrument at a second carrier gas flow rate.